1. Field of the Invention
The present invention relates to photosensitive sensor groups that comprise vertically stacked sensors. In each group, semiconductor material chromatically filters incident electromagnetic radiation vertically (optionally, other material also filters the radiation) and each sensor simultaneously detects a different wavelength band. The invention also relates to arrays of such sensor groups, with each sensor group positioned at a different pixel location.
2. Background of the Invention
The expressions “filter” and “color filter” are used interchangeably herein (including in the claims) in a broad sense to denote an element that selectively transmits or reflects at least one wavelength band of electromagnetic radiation that is incident thereon. For example, one type of filter is a dichroic mirror that both transmits radiation in a first wavelength band and reflects radiation in a second wavelength band. Examples of filters include short wave pass filters, long wave pass filters, and band pass filters.
The term “radiation” is used herein to denote electromagnetic radiation.
The expression “top sensor” (of a sensor group) herein denotes the sensor of the group that radiation, incident at the sensor group, reaches before reaching any other sensor of the group. The expression that the sensors of a sensor group are “vertically stacked” denotes that one of the sensors is a top sensor of the group, and that the group has at least one axis (sometimes referred to as a “vertical axis”) that extends through all the sensors. As described below, a vertical color filter (“VCF”) sensor group that embodies the invention preferably includes vertically stacked sensors configured such that the group's top sensor has a top surface that defines a normal axis (e.g., is at least substantially planar), and when radiation propagating along a vertical axis of the group is incident at the group, the radiation is incident at the top sensor with an incidence angle of less than about 30 degrees with respect to the normal axis (e.g., the radiation is normally incident at the group).
The expression used herein that two elements, included in a structure having a vertical axis, are “laterally” (or “horizontally”) separated denotes that there is an axis parallel to the vertical axis that extends between the elements but intersects neither element.
The expression that an item “comprises” an element is used herein (including in the claims) to denote that the item is or includes the element.
MOS active pixel sensors are known in the art. Multiple-wavelength band active pixel sensor arrays are also known in the art. One type of multiple-wavelength band active pixel sensor array employs red, green, and blue sensors disposed horizontally in a pattern at or near the semiconductor surface. Color overlay filters are employed to produce the color selectivity between the red, green, and blue sensors. Such sensors have the disadvantage of occupying a relatively large area per resolution element as these sensors are tiled together in a plane. In addition, reconstruction of a color image from such a sensor array is computationally intensive and often results in images with artifacts, defects, or inferior resolution.
Another type of multiple-wavelength band pixel sensor array employs groups of sensors, each group including sensors in a vertically-oriented arrangement. An example of an early multiple-wavelength vertical sensor group for detecting visible and infra-red radiation is disclosed in U.S. Pat. No. 4,238,760 to Carr, in which a first diode in a surface n-type epitaxial region is responsive to visible light and a second diode (including a buried p-region in an underlying n-type substrate) is responsive to infrared radiation. Carr teaches that contact to the buried diode is made using a deep diffusion process “similar to diffusion-under-film collector contact diffusion common in bipolar IC processing and for reducing the parameter RCS.” Carr also discloses an embodiment in which a V-groove contact (created by a process that includes a step of etching through the n-type epitaxial region) provides contact to the buried p-type region. The disclosed device has a size of 4 mils square. The device disclosed in the Carr patent has several shortcomings, the most notable being its large area, rendering it unsuitable for the image sensor density requirements of modern imaging systems. The technology employed for contact formation to the buried infrared sensing diode is not suitable for modern imaging technology or extension to a 3-color sensor.
U.S. Pat. No. 5,965,875 to Merrill discloses a three-color, visible light, sensor group in which a structure is provided using a triple-well CMOS process wherein the blue, green, and red sensitive PN junctions are disposed at different depths relative to the surface of the semiconductor substrate upon which the imager is fabricated. This three-color sensor group permits fabrication of a dense imaging array because the three colors are sensed over approximately the same area in the image plane. However, its structure has several shortcomings. First, the sensor group uses a reverse-polarity central green-sensitive PN junction, requiring modified circuits or voltage ranges, possibly involving PMOS transistors in addition to the usual NMOS transistors, to sense and read out the green channel. This requirement disadvantageously increases sensor area and complicates support circuits in detectors that include the sensor groups. The added circuit complexity makes it difficult to make an image sensor array that has flexible color readout capabilities (as disclosed herein) and makes it impossible to achieve the small sensor size required by many modern electronic imaging applications.
U.S. Pat. No. 6,111,300 to Cao, et al., discloses a color active pixel sensor which uses a PIN photodiode to attempt to collect blue light, and two additional semiconductor junction diodes (vertically spaced from the PIN photodiode) within a semiconductor substrate to detect green and red light. Among the shortcomings of this sensor are difficult and non-standard manufacturing techniques, use of structures that prohibit high density of sensors (in an array), no ability to select different colors for read out, and inability to perform detection of three or more colors using a monolithic semiconductor substrate.
Findlater et al. (“A CMOS Image Sensor Employing a Double Junction Photodiode,” K. M. Findlater, D. Renshaw, J. E. D. Hurwitz, R. K. Henderson, T. E. R. Bailey, S. G. Smith, M. D. Purcell, and J. M. Raynor, in 2001 IEEE Workshop on Charge-Coupled Devices and Advanced Image Sensors, IEEE Electron Devices Society (2001)) disclose an active pixel sensor that employs a double-junction photodiode in conjunction with an organic filter overlay. Each double-junction photodiode comprises top and bottom p-type layers with an n-type layer between them. The n-type layer forms the cathode of a first photodiode, the bottom p-type layer forms the anode of a second photodiode, the first photodiode is coupled to a first readout circuit, and the second photodiode is coupled to a second readout circuit. A mosaic of cyan and yellow filters overlays an array of the sensors so that in each row of the array, the even-numbered sensors receive a radiation in a first wavelength band (blue and green) and the odd-numbered sensors receive radiation in a second wavelength band (red and green). The performance of an array of such sensors is limited by the poor color response of the double-junction photodiode and by the fact that the n-well forms the cathode of both photodiodes, making the sensor design very susceptible to non-linear crosstalk between the color channels. Additionally, the authors cite non-uniformity and process/fabrication constraints that limit the performance and potential benefits of this design.
Several types of vertical color filter (“VCF”) sensor groups and methods for fabricating them are described in U.S. patent application Ser. No. 09/884,863, filed Jun. 18, 2001, and in above-referenced U.S. patent application Ser. No. 10/103,304. A VCF sensor group includes at least two photosensitive sensors that are vertically stacked with respect to each other (with or without non-sensor material between adjacent sensors). Each sensor of a VCF sensor group has a different-spectral response. Typically, each sensor has a spectral response that peaks at a different wavelength. In some embodiments, a VCF sensor group (or one or more of the sensors thereof) includes a filter that does not also function as a sensor.
A VCF sensor group simultaneously senses photons of at least two wavelength bands in the same area of the imaging plane. In contrast, time sequential photon sensing methods do not perform photon sensing at the same time for all wavelength bands. The sensing performed by a VCF sensor group included in an imager occurs in one area of the imager (when the imager is viewed vertically), and photons are separated by wavelength as a function of depth into the sensor group.
Typically, each sensor detects photons in a different wavelength band (e.g., one sensor detects more photons in the “blue” wavelength band than each other sensor, a second sensor detects more photons in the “green” wavelength band than each other sensor, and a third sensor detects more photons in the “red” wavelength band than each other sensor), although the sensor group typically has some “cross-talk” in the sense that multiple sensors detect photons of the same wavelength.
VCF sensor groups can be used for a variety of imaging tasks. In preferred embodiments, they are used in digital still cameras (DSC). However they can be employed in many other systems, such as linear imagers, video cameras and machine vision equipment.
A VCF sensor group uses the properties of at least one semiconductor material to detect incident photons, and also to selectively detect incident photons of different wavelengths at different depths in the group. The detection of different wavelengths is possible due to the vertical stacking of the sensor layers of the sensor group in combination with the variation of optical absorption depth with wavelength in semiconductor materials. The costs of manufacturing VCF sensor groups are substantially reduced because VCF sensor groups do not require external color filters (as are traditionally used in color image sensors) and do not require color filters that are distinct from the sensors themselves (the sensors themselves are made of semiconductor material that itself provides a filtering function). However, in some embodiments of the invention, VCF sensor groups do include (or are used with) color filters that are distinct from the sensors themselves. The spectral response characteristics of VCF color sensor groups typically are much more stable and less sensitive to external factors such as temperature or other environmental factors (that may be present during or after manufacturing) than are conventional color sensors with non-semiconductor based filters.
A VCF sensor group is preferably formed on a substrate (preferably a semiconductor substrate) and comprises a plurality of vertically stacked sensors (e.g., sensor layers) configured by doping and/or biasing to collect photo-generated carriers of a first polarity (preferably negative electrons). The sensors include (or pairs of the sensors are separated by) one or more reference layers configured to collect and conduct away photo-generated carriers of the opposite polarity (preferably positive holes). The sensors have different spectral sensitivities based on their different depths in the sensor group, and on other parameters including doping levels and biasing conditions. In operation, the sensors are individually connected to biasing and active pixel sensor readout circuitry. VCF sensor groups and methods for fabricating them are discussed more fully in U.S. patent application Ser. No. 09/884,863, and in the parent application, U.S. patent application Ser. No. 10/103,304.
An array of VCF sensor groups can be modified by positioning a pattern of color filters over the array, as described in U.S. patent application Ser. No. 10/103,304. Using filters made of only a single filter material and positioned over a subset of the sensor groups, an array with three sensors per sensor group can be operated to detect radiation in four, five, or six different wavelength bands (by reading out signals from different selected subsets of the sensor groups of the array). This can yield improved color accuracy. Any of many different types of filters can be employed, including organic dye filters as in some conventional color image sensors, and filters comprising one or more layers that are integrated with the sensor group by a semiconductor integrated circuit fabrication process (e.g., a layer of polysilicon to absorb short wavelengths, an interference filter that is a stack of alternating oxide and nitride layers, or another interference filter for shaping the spectral response by interference effects).